Concerning a polyester film, in particular, a biaxially-oriented polyester film is used as magnetic recording materials, electrical insulating materials, capacitor materials, packaging materials and construction materials due to their mechanical and electrical properties. Furthermore, the film is also used in a variety of industrial materials in photography, graphic arts and heat-sensitive transcription.
However, a biaxially-oriented polyester film is high in crystalline orientation on the surface and low in adhesive properties with various types of paints and inks, and hence a drawback of the film.
Therefore, various methods have been studied in which adhesive properties are imparted to the surface of polyester film.
Conventionally, there is method for imparting adhesive properties to the surface of polyester film by carrying out various types of good adhesiveness-imparting treatments to the polyester film, which is the base film. For this purpose, many methods have been proposed and studied, for example, a surface activation method in which the surface is treated by corona discharge, ultraviolet irradiation or plasma is given to the film surface, a surface etching method in which chemicals such as acid, alkaline or amine aqueous solutions are used, and a method in which various types of resins with adhesive properties such as acrylic resin, polyester resin, urethane resin or polyolefin resin are provided on the film surface as a primer layer.
A resin layer made with a polyester resin and an oxazoline-based cross-linking agent has been proposed as an good adhesive primer layer with excellent solvent resistance and blocking resistance, for example, on page 5 in the Japanese Published Unexamined Patent Application No. Hei 02-60941, in Claim 1 of the Japanese Published Unexamined Patent Application No. Hei 06-293838, in Claim 1 of the Japanese Published Unexamined Patent Application No. Hei 06-293839, in Claim 1 of the Japanese Published Unexamined Patent Application No. Hei 07-242758, in Claim 1 of the Japanese Published Unexamined Patent Application No. 2004-243760, in Claim 1 of the Japanese Published Unexamined Patent Application No. 2003-136660 and others.
However, a polyester film easily soften or melt when heated and also burn, and hence a drawback.
Increasing requests to make a polyester film flame-retardant have been made due to the possibility of fire, in particular when the film is used in copper-clad laminated plates, adhesive tape, flexible print circuit substrates, membrane switches, film heaters and flat cables as electrical insulating materials or construction materials.
Furthermore, when a polyester film is used as a base film for copper-clad laminated plates to be used as flexible print substrates and the like, the film is required to have layer-to-layer adhesiveness which justifies its use, in addition to the above-described flame-retardant properties.
As disclosed on page 1 to 2 in the Japanese Published Unexamined Patent Application No. Hei 10-278206, a polyester film have been technically improved in regard to flame-retardant properties through the conventional method in which flame-retardants based on bromine, phosphorus or inorganic chemicals are kneaded into a polyester film or in which halogen-including components or phosphorus-including components are subjected to copolymerization.
Furthermore, as disclosed in Claim 1 of the Japanese Published Unexamined Patent Application No. 2004-243760 and in Claim 1 of the Japanese Published Unexamined Patent Application No. 2003-80651, a method has been proposed in which resins such as polyamic acid are laminated onto a polyester film, thereby imparting heat resistance and flame-retardant properties to the film.
However, even when the technology is used which has been disclosed on page 5 in the Japanese Published Unexamined Patent Application No. Hei 02-60941, in Claim 1 of the Japanese Published Unexamined Patent Application No. Hei 06-293838, in Claim 1 of the Japanese Published Unexamined Patent Application No. Hei 06-293839, in Claim 1 of the Japanese Published Unexamined Patent Application No. Hei 07-242758, in Claim 1 of the Japanese Published Unexamined Patent Application No. 2004-243760, in Claim 1 of the Japanese Published Unexamined Patent Application No. 2003-136660 and others, as described above, it is difficult to obtain sufficient adhesiveness when a polyester film is subjected to heat treatment for long periods of time at high temperatures when and/or after a laminating material is provided, and in particular when a laminating material such as metal that is different in dimensional stability under heating from a polyester film is laminated for long periods of time at high temperatures.
For example, when metal is adhered to resin, in general, an adhesive agent made with resin components is used to effect adhesion. However, it is not easy to allow metal to adhere to resin in a short time at low temperatures because they are different in raw materials. In order to attain an excellent adhesiveness, they must be adhered for long periods of time at high temperatures. In particular, when copper foil and a resin film are used to produce a copper-clad laminated plate which is used as a circuit substrate for electrical appliances and electronic devices, strong adhesive properties exceeding 80 g/mm is needed. In order to attain this level of adhesiveness, it is necessary to meet the conditions that adhesion is made for long periods of time at high temperatures.
However, where a material such as metal that is different in dimensional stability under heating (heat shrinkage ratio and thermal expansion factor) from a polyester film is adhered by using an adhesive agent for long periods of time at high temperatures, there is a problem that the dimension changes on adhesion treatment at high temperatures or on cooling after the adhesion treatment due to differences in dimensional stability under heating, resulting in interlayer stress, which subsequently causes poor adhesiveness.
Where a material such as metal that is different in dimensional stability under heating from a polyester film is required to be strongly adhered, good adhesive primer layers described on page 5 in the Japanese Published Unexamined Patent Application No. Hei 02-60941, in Claim 1 of the Japanese Published Unexamined Patent Application No. Hei 06-293838, in Claim 1 of the Japanese Published Unexamined Patent Application No. Hei 06-293839, in Claim 1 of the Japanese Published Unexamined Patent Application No. Hei 07-242758, in Claim 1 of the Japanese Published Unexamined Patent Application No. 2004-243760, in Claim 1 of the Japanese Published Unexamined Patent Application No. 2003-136660 or others are not able to solve the problem, even if provided on a polyester film.
The above-described problem of insufficient adhesiveness is often found particularly when a polyester film is peeled from a boundary face with another layer. Where an good adhesive primer layer is provided on the polyester film, peeling may often develop on a boundary face between the polyester film and a primer layer. Therefore, it is important to improve adhesive properties on the boundary face.
It is also difficult to attain adhesive properties and solvent resistance together. The adhesive properties are the properties of carrying out heat treatment for long periods of time at high temperatures.
Since the invention disclosed in the Japanese Published Unexamined Patent Application No. Hei 10-278206 (on page 1 to 2) is that in which a flame-retardant is added to a polyester film, or polyester is subjected to copolymerization with halogen-including components or phosphorus-including components, there is a problem that intrinsic mechanical properties of the polyester film are reduced. Furthermore, there are concerns that, for example, halogen compounds may develop dioxin and other harmful chemicals, depending on combustion conditions, thereby affecting the environment or contaminating processes, which cause problems.
Furthermore, the technology disclosed in the Japanese Published Unexamined Patent Application No. 2004-243760 (Claim 1 and others) and the Japanese Published Unexamined Patent Application No. 2003-80651(Claim 1 and others) has solved such a problem that flame-retardant properties are imparted without reduction in intrinsic mechanical properties of polyester film. The technology has also improved adhesive properties of the polyester film with resins such as polyamic acid by the use of a good adhesive primer layer.
However, the good adhesive primer layers disclosed in the Japanese Published Unexamined Patent Application No. 2004-243760 (Claim 1 and others) and the Japanese Published Unexamined Patent Application No. 2003-80651(Claim 1 and others) have failed in providing a sufficient adhesiveness in a case where a laminated material such as metal that is different in dimensional stability under heating from a polyester film is laminated for long periods of time at high temperatures. For example, where a resin such as polyamic acid is laminated on the good adhesive primer layer and thereafter copper foil is laminated on the resin such as polyamic acid for long periods of time at high temperatures, a sufficient adhesiveness is not obtained.
In this instance as well, the problem of an insufficient adhesiveness is more often found where peeling develops on a boundary face between polyester film and an good adhesive primer layer. It is also important to improve adhesive properties on the boundary face.